In this study, the effect of naloxone in the prevention of experimental stress ulcers was investigated. A prophylactic effect of naloxone on stress ulcer formation was not observed. On the contrary, naloxone increased the gastric lesion score. Naloxone did neither decrease gastric acidity in stress nor in non-stress conditions. Under non-stress conditions naloxone did not change gastric mucosal barrier by reducing Na+ and K+ ion concentrations of the gastric secretion. Under stress conditions the barrier could not be maintained. Naloxone decreased gastric mucus production under stress.